This invention relates generally to devices and methods for the measurement of wavelengths of light or the detection of light at selected wavelengths.
U.S. Pat. Nos. 5,838,437 and 5,892,582 described optical wavelength scanners and spectrum analyzers which employ tunable Fiber Fabry-Perot filters (FFP-TF) for wavelength scanning. The devices and systems described in these patents incorporate a multiwavelength reference for in-situ calibration of the FFP-TF for wavelength measurement. The use of the multiwavelength reference minimizes the effects of drift and nonlinearity in the FFP-TF scanner. In a specific embodiment, the multiwavelength reference is a combination of a fixed fiber Fabry-Perot filter which provides a comb of wavelengths of known separation and a reference fiber Bragg grating (FBG) which provides a reference peak or notch to identify the wavelength of a peak in the comb. The comb of references provides reference peaks over the wavelength range of interest for detection or analysis. U.S. Pat. Nos. 5,838,437 and 5,892,582 are incorporated by reference herein in their entirety.
This invention provides improved devices for highly accurate wavelength detection which employ a pre-calibrated FFP-TF obviating the need for in situ calibration of the filter, significantly simplifying the devices which generally have fewer components and simplified hardware and software, decreasing the cost of the devices, and increasing the speed of measurement without significant loss in wavelength accuracy. The improved devices can be used in a variety of optical applications including tunable receivers, sensor interrogators, wavelength meters, optical tracking filters and optical channel analyzers. The devices of this invention generally avoid the use of optical switches needed in prior devices employing in situ calibration to allow comparison of reference and measured wavelengths. A one time calibration of the FFP-TF is performed at a range of temperatures over the intended operating temperature range of the filter to generate a set of calibration coefficients for curve fitting. These coefficients, which embody correction data for wavelength and power error, are used to correct wavelength and power measurements in devices using the FFP-TF scanner. FFP-TF can also be precalibrated for bandwidth variation. The calibration can be performed once (at multiple temperatures) after the device is constructed rather than periodically within the instrument in the field.
FFP-TF employ piezoelectric actuators or transducers (PZTs) to change the length of the FP cavity and thereby tune the wavelength of the filter. PZTs exhibit dynamic nonlinearities arising from nonlinear length dependence upon voltage, voltage hysterisis, and temperature. PZTs retain a memory, in the form of remnant polarization, of the voltage and temperature conditions to which they have been exposed. In particular, after exposure of a PZT to very low temperatures, it can take up to a month for the PZT to return to its original steady state polarization condition. Because of this sensitivity to voltage and temperature conditions, in situ calibration, as described in the U.S. patents noted above, was believed to be necessary to obtain wavelength accuracy in the 10-20 picometer range desirable for applications noted herein. The inventors have discovered that application of a low level negative voltage to the PZTs used in the FFP-TFs, rapidly resets the PZT to its original steady state condition eliminating remnant polarization due to the voltage or temperature history of the PZT. The set of calibration coefficients determined for a FFP-TF with the PZTs in this steady state condition can then be employed at any time in the future, if the PZT of the FFP-TF is reset to the steady state condition prior to making wavelength measurement and applying the pre-determined calibration coefficients.
The length of PZTs are typically changed by application of a positive variable voltage to the PZT. In a low voltage PZT, the range of voltage applied to change the length ranges from 0 to about 40 volts. An FFP-TF is typically tuned through a wavelength range by application of a voltage ramp to the PZT of the filter. Calibration of the FFP-TF associates a voltage applied to the PZT to the wavelength passed by the filter at that applied voltage.
The inventors have found that application of a low negative voltage, e.g., xe2x88x925 volts to the PZT of the FFP-TF (a stacked PZT) resets the PZT to the original steady state condition eliminating remnant polarization within about 1 minute. After the PZT is reset, application of the predetermined calibration coefficients provides reproducible, accurate wavelength calibration of the filter. The negative reset voltage employed is preferably less in magnitude than about 25% of the depoling voltage (typically about 40 volts) of the PZT, i.e., less than about 10 V in magnitude. The resetting procedure has demonstrated excellent stability over a wide range of temperatures.
The devices of this invention can be programed to apply the resetting voltage to the PZTs of the FFP-TF whenever the device is turned on. Devices can also be equipped with a controller and voltage source that allows application of the negative reset voltage periodically when the device is in operation, selectively as determined by the operator of the device, or in response to an event or condition, such as the detection of a loss in wavelength accuracy or a change in operating conditions.
The invention provides a calibration method for tunable optical filters which is particularly useful with Fiber Fabry-Perot Tunable Filters (FFP-TFs) and specifically useful with FFP-TFs which employ piezoelectric transducers as tuning elements. The method is generally applicable to achieve wavelength error of less than about xc2x150 picometers over the operating temperatures of the filter. Preferably, application of the calibration method achieves wavelength error of less than about xc2x120 picometers over the operating temperature range of the filter. The invention also provides tunable optical filters calibrated by the inventive method and optical devices for measurement of wavelengths of light which comprise the inventive calibrated tunable optical filters.
The calibration method involves the determination of calibration coefficients employing a plurality of known wavelengths of light over a wavelength region of interest to generate a set of calibration coefficients.
A set of calibration coefficients is determined at each of a plurality of temperatures over the operating temperature range of the filter. The operating temperature range of the filter may, for example, range from 0xc2x0 C. to about 60xc2x0 C. In a preferred embodiment sets of calibration coefficients are determined at intervals of about 1xc2x0 C. to about 10xc2x0 C. over the operating temperature range of the device. For example, a set of calibration coefficients over the desired wavelength range spanned by the plurality of known wavelengths is determined for each interval of 1xc2x0 C., 5xc2x0 C. 10xc2x0 C. over the operating temperature range of the filter.
The sets of calibration coefficients determined for the tunable filter which span the wavelength region of interest and the operating temperature range of interest are stored in a microprocessor or computer. The stored coefficients are then employed to correct measurements or determinations of unknown wavelengths by the tunable filter.
The stored coefficients can also be used to set the tunable filter to detect the presence of a selected wavelength among a plurality of wavelengths such as in a broad band of wavelengths.
To correct a wavelength measured by the tunable filter at a selected temperature, a set of coefficients determined at the selected temperature or within about 1xc2x0 C. to about 10xc2x0 C. of the selected temperature is employed. Where no set of coefficients is determined at the selected temperature, it is preferred to correct the wavelength measurement by interpolation employing two sets of coefficients measured at temperatures which bracket the selected temperature.
In a specific embodiment, the calibration method of this invention is applied to tunable filters which employ piezoelectric transducers as tuning elements. In these filters, the length of the piezoelectric transducer is changed to tune the wavelength of the filter. In a preferred application of the calibration method to such filters, a low negative voltage is applied to the piezoelectric transducer prior to determination of calibration coefficients and prior to the measurement of wavelength using the filter.
The invention also provides optical devices-for the measurement or detection of wavelengths of light or for the selection of a wavelength of light from a plurality of wavelengths of light. These devices comprise a tunable filter, preferably a fiber Fabry-Perot tunable filter, calibrated by the method of this invention. The device comprises the tunable filter and some means for storing the sets of calibration coefficients and employing the sets of calibration coefficients to correct wavelength measurements by the filter. The sets of calibration coefficients may also be used to tune the filter, e.g., by adjusting the voltage applied to a piezoelectric transducer, to receive or pass a selected wavelength.